The invention is particularly well suited for use in the recapturing of heat from hot flue gases to preheat combustion air employed in the apparatus from which the flue gases exit. Flue gases from many industrial furnaces utilizing fossil fuels, such as natural gas, oil or coal, leave the furnace at high temperatures which are slightly above temperatures of the furnace. The heat in flue gases of a typical industrial installation amounts to 30-80 percent of the heat in the furnace. Thus, it is important to recapture as much heat as possible from the exiting flue gases for recycling to the furnace, by using the recaptured heat to preheat the combustion air normally used in the furnace. This is achieved by using devices known as air preheaters or recuperators.
The combustion air of a typical recuperator is passed through a metal or ceramic pipe which is positioned within a stream of hot flue gases. The heat of the hot flue gases is first transferred to the tube and then to the combustion air being circulated in the tube. In such designs, the heat transfer from the hot flue gas to the tube and then to the air, is primarily by convection which is limited by the gas velocity which, in turn, determines the pressure drop across the flue gas side of the recuperator. A high pressure drop on the flue gas side creates within the furnace, a positive pressure which causes problems in the operation and maintenance of the furnace. Another limitation of such recuperators is the fixed ratio of surface area to cross-sectional area of conventional tubes or pipes used in the recuperators. It should be readily apparent that the ratio of the heat transfer surface area to the cross-sectional area of a given size pipe having a circular cross-section, cannot be radically changed, because the cross-sectional area increases as the surface area increases thereby resulting in a lower velocity of the air being circulated in the pipe and consequently a lower coefficient of heat transfer. The invention is designed to overcome such problems.
Briefly stated, the invention is in a device which is disposable in a flue for recapturing heat from flue gases exiting the flue. The device essentially comprises a pair of tubes which are composed of heat conductive material. Means are provided for mounting the tubes in parallel relation longitudinally of the flue, and for circulating a fluid, e.g. air, under pressure, through the tubes. A shield having at least heat radiating surfaces facing the tubes, is disposed longitudinally between the tubes for radiating against the tubes, heat which is received from the hot flue gases.